Calculating Velocity of Wreckage After Collision

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The discussion focuses on calculating the velocity of wreckage after a perfectly inelastic collision between two cars of different masses and speeds. Participants emphasize the importance of using conservation of momentum to determine the combined velocity post-collision. One user expresses difficulty in applying the conservation of momentum correctly, while another clarifies that only momentum conservation is necessary, not energy conservation. The correct approach involves setting up two equations: one for momentum and another reflecting the inelastic nature of the collision. The conversation highlights the need for a solid understanding of these principles to solve collision problems effectively.
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Homework Statement


A car of mass 950kg traveling at 60km/h in an easterly direction collided perfectly inelastically with another care of mass 1250kg traveling at 55.0km/h in a northerly direction. The design of the smaller care incorporated safety features which ensured that any occupants came to rest relative to the care in a tie of 250m/s. Both vehicles had one occupant of 75.0kg

calculate the velocity of the wreckage immediately after the collision.

Use conservation of momentum to determine the change in momentum of both vehicles



Homework Equations


m1v1 + m2v2 = m'1v'1 + m'2 v'2
I am not sure if the kinetic energy equation fits here too?
.5 m1 v1 E2 + .5 m2 v2 E2 = .5 m1 v'1E2 + .5 m2v'2E2
r= square root (xE2 +yE2)
p=mv

The Attempt at a Solution



I converted all the km/h to m/s
I calculated the momentum of each vehicle p=mv (I also included the weight of the driver for each). Using the answers I found the resultant.
I then rearranged the equation to find v and got the right answer in it.

I am now really struggling with the conservation of momentum - these always get me out. I can't seem to use the right equation and am just blank.

Can someone let me know = thank you. :)
 
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Hi Chica1975! :smile:
Chica1975 said:
I am now really struggling with the conservation of momentum - these always get me out. I can't seem to use the right equation and am just blank.

As I said in the other thread …
tiny-tim said:
You always need two equations to solve a collision question … one is always conservation of momentum, the other is either conservation of energy or some other constraint like "perfeclty inelastic" or " coefficient of restitution = 0.5".

In this case, your second equation (which you did use) is vf1 = vf2 …

you don't need, and mustn't use, conservation of energy! :wink:

(But you always need conservation of momentum)
 

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